The invention relates to a high-strength, air-hardenable, multi-phase steel with excellent processing properties. Advantageous refinements are the subject of dependent claims.
The invention also relates to a method for producing a hot-rolled and/or cold-rolled strip from such a steel and its heat treatment by means of air-hardening and, optionally, subsequent tempering, and a steel strip produced by this method.
The invention relates in particular to steels having a tensile strength in the range of at least 950 MPa in the non-annealed state for the production of components which have improved deformability (such as increased hole expansion and increased bending angles) and improved weld properties.
By heat treating these steels according to the invention, the yield strength and tensile strength can be increased, for example, by air-hardening with optional subsequent tempering.
The hotly contested automotive market forces manufacturers to constantly find solutions to reduce fleet consumption and CO2 emissions, while maintaining the greatest possible comfort and occupant protection. On the one hand, the weight reduction of all vehicle components plays a decisive role but on the other hand also the optimal behavior of the individual components under conditions of high static and dynamic stress both during use and in the event of a crash.
By providing high-strength to ultra-high-strength steels and reducing the thickness of the sheet metal, the weight of the vehicles can be reduced while simultaneously improving the forming characteristics and component properties during manufacture and operation.
Therefore, high-strength to ultra-high-strength steels must meet comparatively high requirements with respect to their strength and ductility, energy absorption and processing, such as, for example, during punching, hot and cold forming, hot tempering (e.g. air-hardening, press-hardening), welding and/or surface treatment, e.g. a metallic refinement, organic coating or varnishing.
Therefore, in addition to the demanded reduction in weight through reduced sheet thicknesses, newly developed steels must meet the increasing requirements placed on materials such as yield strength, tensile strength, solidification behavior and elongation at break while also possessing good processing properties such as deformability and weldability.
Therefore, when reducing the sheet thickness as mentioned above, a high-strength to ultra-high-strength steel with a single-phase or multi-phase microstructure has to be used to ensure sufficient strength of the motor vehicle components and to meet the high requirements placed on component in terms of tenacity, edge crack resistance, improved bending angle and bending radius, energy absorption and hardening capacity, and Bake Hardening Effect.
There is also an increasing demand for improved suitability for joining in the form of better general weldability, such as a larger usable welding area when using resistance spot welding and an improved failure behavior of the weld seam (fracture pattern) under conditions of mechanical stress, as well as a sufficient resistance to delayed hydrogen embrittlement (i.e., delayed fracture free). The same applies to the suitability for welding of ultra-high-strength steels in the production of pipes, which are produced, for example, by means of the High-Frequency Induction welding method (HFI).
The hole expansion capacity is a material property which describes the resistance of the material against the risk of fracture and crack propagation during forming operations in areas close to the edge, such as for example, during collar forming.
The hole expansion test is, for example, governed by the normative standard ISO 16630. Prefabricated holes, for example, punched into a sheet, are then expanded by means of a mandrel. The measured value is the change in the hole diameter relative to the starting diameter, at which the first crack occurs through the sheet at the edge of the hole.
Improved edge crack resistance means increased deformability of the sheet edges and can be described by an increased hole expansion capacity. This is known under the synonyms “Low Edge Crack” (LEC) and “High Hole Expansion” (HHE) as well as Xpand®.
The bending angle describes a material property which allows drawing conclusions regarding the material behavior during forming operations with dominant bending processes (for example, during folding) or also when subjected to crash loads. Increased bending angles therefore increase the passenger compartment safety. The determination of the bending angle (α) is governed by the plate bending test set forth in the normative standard VDA 238-100.
The above mentioned characteristics are important for components which, prior to heat treatment, for example air hardening with optional tempering, can be formed into very complex components.
Improved weldability, as is known, is achieved inter alia by a reduced carbon equivalent. Synonyms therefore are for example “underperitical” (UP) or the already known “Low Carbon Equivalent” (LCE). Hereby the carbon content is typically less than 0.120% by weight. Furthermore, the failure behavior or the fracture pattern of the weld seam can be improved by alloying with micro-alloying elements.
Components of high strength must have sufficient resistance against hydrogen induced material embrittlement. Testing of Advanced High Strength Steels (AHSS) used in automotive production for resistance against production-related hydrogen-induced brittle fractures is governed by SEP1970 and is tested via the bent beam test and the perforation tensile test. In vehicle construction, dual-phase steels are increasingly used which consist of a ferritic basic microstructure into which a martensitic second phase is incorporated. It has been found that in the case of low-carbon, micro-alloyed steels, proportions of further phases such as bainite and residual austenite have an advantageous effect for example on the hole expansion behavior, the bending behavior and the hydrogen-induced brittle fracture behavior. The bainite can hereby be present in various forms, e.g. upper and lower bainite.
The specific material characteristics of the dual-phase steels, such as low yield ultimate ratio in association with very high tensile strength, strong strain hardening and good cold formability, are well known, but are often no longer sufficient with ever more complex component geometries.
In general, the group of multi-phase steels is increasingly used. The multi-phase steels include, for example, complex-phase steels, ferritic-bainitic steels, TRIP-steels, as well as the dual-phase steels described above, which are characterized by different microstructural compositions.
Complex phase steels are, according to EN 10346, steels which contain small proportions of martensite, residual austenite and/or perlite in a ferritic/bainitic basic microstructure, wherein a strong grain refinement is caused by a delayed recrystallization or precipitation of microalloying elements.
Compared to dual phase steels these complex phase steels have higher yield strengths, a higher yield ultimate ratio, a lower strain hardening and a higher hole expansion capacity.
Ferritic-bainitic steels are, according to EN 10346, steels containing bainite or work hardened bainite in a matrix of ferrite and/or work-hardened ferrite. The strength of the matrix is caused by a high dislocation density, by grain refining and the precipitation of micro-alloying elements.
Dual-phase steels are, according to EN 10346, steels with a ferritic basic microstructure, in which a martensitic second phase is incorporated in the form of islands, in some cases also with portions of bainite as the second phase. Dual-phase steels have a high tensile strength, while also exhibiting a low yield ultimate ratio and strong strain hardening.
TRIP-steels are, according to EN 10346, steels with a predominantly ferritic basic microstructure in which bainite and residual austenite are incorporated, which can transform into martensite during deformation (TRIP effect). Because of its strong strain hardening, the steel achieves high values of uniform elongation and tensile strength. Combined with the bake hardening effect, high component strengths can be achieved. These steels are suitable for stretch forming as well as for deep drawing. However, higher sheet metal holding forces and pressing forces are required during forming of the material. Comparatively strong rebounding must be taken into account.
High-strength steels with single-phase microstructure include for example bainitic and martensitic steels.
Bainitic steels are, according to EN 10346, characterized by a very high yield strength and tensile strength with a sufficiently high elongation for cold forming processes. Their chemical composition results in good weldability. The microstructure is typically composed of bainite. Small proportions of other phases, e.g. martensite and ferrite may be contained in the microstructure.
Martensitic steels are, according to EN 10346, steels which contain small proportions of ferrite and/or bainite in a basic microstructure of martensite as a result of thermo-mechanical rolling. This steel grade is characterized by a very high yield strength and tensile strength with a sufficiently high elongation for cold forming processes. Within the group of multi-phase steels, the martensitic steels have the highest tensile strength values. The suitability for deep drawing is limited. The martensitic steels are mainly suitable for bending forming processes, such as roll forming.
Heat treatable steels are, according to EN 10083, steels which achieve a high tensile strength and durability by heat treatment (=quench-hardening and tempering). When the cooling during hardening at air results in bainite or martensite, the method is referred to as “air-hardening”. Via tempering after the hardening the strength/toughness ratio can be influenced in a targeted manner.
Areas of Application and Production Processes
High-strength and ultra-high-strength multi-phase steels are used, inter alia, in structural, chassis and crash-relevant components, as sheet metal plates, tailored blanks as well as flexible cold rolled strips, so-called TRB®s or tailored strips.
The Tailor Rolled Blank lightweight technology (TRB®) enables a significant weight reduction by means of a load-adapted sheet thickness over the component length and/or steel grade.
In the continuous annealing plant, a special heat treatment takes place for adjusting a defined microstructure, wherein for example comparatively soft constituents, such as ferrite or bainitic ferrite, result in a low yield strength of the steel, and hard constituents of the steel, such as martensite or carbon-rich bainite contribute to the strength of the steel.
For economic reasons, cold-rolled high-strength to ultra-high-strength steel strips are usually annealed in the continuous annealing process to a readily formable metal sheet. Depending on the alloy composition and the strip cross-section, the process parameters such as throughput speed, annealing temperatures and cooling rate (cooling gradients) are adjusted according to the required mechanical-technological properties with the microstructure required therefore.
For adjusting a dual-phase microstructure, the pickled hot strip, in typical thicknesses between 1.50 to 4.00 mm, or cold strip, in typical thicknesses of 0.50 to 3.00 mm, is heated in the continuous annealing furnace to such a temperature that the required microstructure forms during recrystallization and cooling.
Particularly in the case of different thicknesses in the transition region from one strip to the other it is difficult to achieve a constant temperature. In the continuous annealing of alloy compositions with process windows that are too narrow, this can lead to the fact that, for example, the thinner strip is either moved too slowly through the furnace, thereby lowering the productivity or the thicker strip is moved too quickly through the furnace thereby failing to achieve the required annealing temperatures and cooling gradients for achieving the desired microstructure. The consequences are increased rejects and high error costs.
Widened process windows are necessary so that, given the same process parameters, the required strip properties can be achieved even in the case of larger cross-sections of the strips to be annealed.
The problem of a very narrow process window is particularly pronounced in the annealing when load-optimized components are to be produced from hot-rolled or cold-rolled strip which have varying strip thicknesses over the strip length and width (for example, by flexible rolling).
However, when strongly varying sheet thicknesses are involved, production of TRB®s with multi-phase microstructure employing the presently known alloys and available continuous annealing systems requires increased costs, for example an additional heat treatment prior to the cold rolling. In regions of different sheet thickness, i.e., in case of varying degrees of rolling reduction, a homogenous multi-phase microstructure cannot be established in cold-rolled and hot-rolled steel strips due to the temperature difference in the conventional alloy-specific narrow process windows.
A method for producing a steel strip of different thickness over the strip length is e.g. described in DE 100 37 867 A1.
When high demands on corrosion protection require the surface of the hot or cold strip to be hot dip galvanized, the annealing is usually carried out in a continuous annealing furnace arranged upstream of the hot dip galvanizing bath.
Also in the case of hot strip, depending on the alloy concept, the demanded microstructure is not established until annealing in the continuous furnace, in order to realize the demanded mechanical properties.
Deciding process parameters are thus the adjustment of the annealing temperatures and the speed, but also the cooling rate (cooling gradient) in the continuous annealing because the phase transformation is temperature and time dependent. Thus, the less sensitive the steel is regarding the uniformity of the mechanical properties when the temperature and time course changes during the continuous annealing, the greater is the process window.
In continuous annealing of hot-rolled or cold-rolled steel strips of different thickness with the alloying concepts for a dual-phase steel known, for example, from laid open patent documents EP 2 028 282 A1, WO 2013/113304 A2 or EP 2 031 081 A1, the problem is that with these alloy compositions, Mechanical properties, but only a narrow process window for the annealing parameters is present in order to be used for cross-sectional jumps, e.g. in the case of width or thickness changes, it is possible to adjust uniform mechanical properties over the strip length without adapting the process parameters.
When using the known alloy concepts, the narrow process window makes it already difficult during the continuous annealing of strips with different thicknesses to establish uniform mechanical properties over the entire length and width of the strip.
In the case of flexibly rolled cold strip made of multi-phase steels of known compositions, the too narrow process window either causes the regions with lower sheet thickness to have excessive strengths resulting from excessive martensite proportions due to the transformation processes during the cooling, or the regions with greater sheet thickness achieve insufficient strengths as a result of insufficient martensite proportions. Homogenous mechanical-technological properties across the strip length or width can practically not be achieved with the known alloy concepts in the continuous annealing.
The goal to achieve the resulting mechanical-technological properties in a narrow region across the strip width and strip length through controlled adjustment of the volume proportions of the microstructure phases has highest priority and is therefore only possible through a widened process window. The known alloy concepts for multiphase steels are characterized by a too narrow process window and are therefore not suited for solving the present problem, in particular in the case of flexibly rolled strips. With the alloy concepts known to date only steels of one strength class with defined cross sectional regions (sheet thickness and strip width) can be produced, hence requiring different alloy concepts for different strength classes or cross sectional ranges.
Steel production has seen a trend towards reducing the carbon equivalent to achieve improved cold processing (cold rolling, cold forming) and improved performance.
However, also the suitability for welding, characterized among other things by the carbon equivalent, is an important evaluation factor.
For example, in the following carbon equivalentsCEV(IIW)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5CET=C+(Mn+Mo)/10+(Cr+Cu)/20+Ni/40PCM=C+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B
the characteristic standard elements, such as carbon and manganese, as well as chromium or molybdenum and vanadium (contents in % by weight) are taken into account.
Silicon plays only a subordinate role in the calculation of the carbon equivalent. This is of crucial importance with respect to the invention. The lowering of the carbon equivalent through lower contents of carbon as well as of manganese is to be compensated by increasing the silicon content. Thus the edge crack resistance and welding suitability are improved while maintaining same strengths.
A low yield ultimate ratio (Re/Rm) in a strength range above 950 MPa in the initial state is typical for a dual-phase steel and serves in particular the formability in drawing and deep drawing operations. This provides the constructor with information regarding the distance between ensuing plastic deformation and failing of the material at quasi-static load. Correspondingly lower yield strength ratios represent a greater safety margin for component failure.
A higher yield ultimate ratio (Re/Rm), as is typical for complex-phase steels, is also characterized by a high resistance against edge cracks. This can be attributed to the smaller differences in the strengths and hardnesses of the individual microstructural constituents and the finer microstructure, which has a favorable effect on a homogeneous deformation in the region of the cutting edge.
With respect to the yield strength as well as the yield ultimate ratio (Re/Rm) there is an overlapping range in the standards, in which an assignment to both complex and dual-phase steels is possible and leads to improved material properties.
The analytical landscape for the achievement of multi-phase steels with minimum tensile strengths of 950 MPa is very diverse and shows very large alloying ranges for the strength-enhancing elements carbon, silicon, manganese, phosphorus, nitrogen, aluminum as well as chromium and/or molybdenum as well as the addition of microalloys such as titanium, niobium, vanadium and boron.
The dimensional spectrum in this strength range is wide and is in the thickness range of about 0.50 to about 4.00 mm for strips which are intended for continuous annealing. The used starting material can be a hot-rolled strip, cold-rolled hot-rolled strip and cold strip. Mainly strips up to a width of about 1600 mm are used, but also slit strips dimensions which result form longitudinal division of the strips. Sheet metals or plates are produced by cutting the strips transversely.
The air-hardenable steel grades known, for example, from EP 1 807 544 B1, WO 2011/000351 and EP 2 227 574 B1, with minimum tensile strengths of 800 (LH®800) and 900 MPa (LH®900), respectively, in a hot-rolled or cold-rolled version are characterized by their very good formability in the soft state (deep drawing properties) and by their high strength after heat treatment (tempering).
During hardening, the microstructure of the steel is transformed into the austenitic range by heating, preferably to temperatures above 950° C. under a protective gas atmosphere. During the subsequent cooling at air or protective gas, a martensitic microstructure is formed for a high-strength component.
Subsequent tempering allows the removal of residual stresses in the hardened component. At the same time, the hardness of the component is reduced so that the required toughness values are achieved.